1. Field of the Invention
The present invention relates to a flat panel display device, and more particularly, to an organic electroluminescent display (OELD) device and method of fabricating an OELD device.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices have been commonly used in flat panel display devices because of their light weight and low power consumption. However, the liquid crystal display (LCD) devices are not light emitting elements, but are light receiving elements that require additional light sources to display images. Accordingly, there is a technical limit for improving brightness, contrast ratio, viewing angle, and enlarging a size of liquid crystal display panels. Thus, research has developed new flat panel display elements that can overcome the aforementioned problems.
Organic electroluminescent display (OELD) devices emit their own light and their viewing angles and contrast ratios are superior compared to the liquid crystal display (LCD) devices. In addition, since OELD device do not require a backlight device to function as a light source, the OELD devices are light weight, have small dimensions, and have low power consumption. Moreover, OELD devices can be driven with low DC (direct current) and have fast response times. Since the OELD devices use solid material instead of fluid material, such as liquid crystal, they are more stable under external impact and have wider operational temperature ranges than the liquid crystal display (LCD) devices. As compared to LCD devices, the OELD devices have relatively low production costs. For example, the OELD devices generally require deposition and encapsulation apparatus, whereas the LCD devices require many different types of fabrication apparatus. In addition, fabrication processes for manufacturing the OELD device are much simpler than the fabrication process for manufacturing the LCD devices.
The OELD devices may be classified into passive matrix-type and active matrix-type devices. In the passive matrix-type OELD devices, pixels are formed in a matrix configuration by crossings of scan and signal lines, wherein the scan lines must be sequentially driven to drive each pixel. Accordingly, a required average luminance depends on a total number of the scan lines. However, in the active matrix-type OELD devices, a thin film transistor (i.e., a switching element) is formed in each sub-pixel to switch the pixel ON and OFF, wherein a first electrode connected to the thin film transistor is turned ON and OFF by the pixel and a second electrode functions as a common electrode.
Moreover, in the active matrix-type OELD devices, a voltage that is supplied to the pixel is stored to a storage capacitor CSt and maintained until a signal for the next frame is applied. Accordingly, the pixel can retain the signal until the next frame regardless of the number of the scan lines. Since the active matrix-type OELD devices can obtain a same luminance with low direct current (DC), the active matrix-type OELD devices are advantageous due to their low power consumption, high resolution, and large size.
FIG. 1 is a schematic circuit diagram of a pixel of an active matrix OELD device according to the related art. In FIG. 1, a scan line 2 is formed along a first direction and signal and power supply lines 4 and 6 are formed along a second direction perpendicular to the first direction. The signal line 4 and the power supply line 6 are spaced apart from each other and define a sub-pixel by crossing the scan line 2, wherein a switching thin film transistor 8 (i.e., an addressing element) is formed at a position near an intersection of the scan and signal lines 2 and 4 and a storage capacitor (CST) 12 is electrically connected to the switching thin film transistor 8 and the power supply line 6. A driving thin film transistor 10 (i.e., a current source element) is electrically connected to the storage capacitor (CST) 12 and the power supply line 6, and an organic electroluminescent diode 14 is electrically connected to the driving thin film transistor 10. Accordingly, if current is supplied to organic light-emitting material of the OELD device along a positive direction, electrons and holes are recombined by passing through a P-N junction between an anode electrode for proving holes and a cathode electrode for proving electrons. The combined electron and the hole have a lower energy state than when the electron and the hole are not recombined and separated away. Accordingly, the OELD device makes use of energy states of the recombined electrons and holes to produce light. In addition, the OELD device can be classified into top emission-type and bottom emission-type OELD devices according to a light emission direction.
FIG. 2 is a cross sectional view of a bottom emission-type OELD device according to the related art. In FIG. 2, a pixel P includes sub-pixels SP for red (R), green (G), and blue (B) colors, wherein first and second substrates 10 and 30 are spaced apart from and oppose each other. A seal pattern 40 is formed on one of the first and second substrates 10 and 30 to attach the first and second substrates 10 and 30 and to prevent liquid crystal material injected between the first and second substrates 10 and 30 from leaking out. A plurality of thin film transistors T and a plurality of first electrodes 12 connected to the thin film transistor is formed within each sub-pixel SP on a transparent substrate 1 of the first substrate 10. An organic light-emitting layer 14 connected to the thin film transistor T is formed on the thin film transistor T and the first electrode 12, wherein the organic light-emitting layer 14 has portions for red (R), green (G), and blue (B) colors corresponding to the first electrode 12. In addition, a second electrode 16 is formed on the organic light-emitting layer 14, wherein the first and second electrodes 12 and 16 serve to supply an electric field to the organic light-emitting layer 14 and the second electrode 16 is spaced apart from the second substrate 30 by the aforementioned seal pattern 40. Although not shown, a moisture absorbent desiccant is formed on an inner side of the second substrate 30 and a semitransparent tape is used to attach the moisture absorbent desiccant to the second substrate 30.
If the first electrode 12 functions as an anode electrode and the second electrode 16 functions as a cathode electrode, then the first electrode 12 is formed of transparent conductive material and the second electrode 16 is formed of material having a low work function. Accordingly, the organic light-emitting layer 14 has a sequential laminated structure of a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and an electron transporting layer 14d. The emission layer 14c has a structure in which light emitting materials for each of the red (R), green (G), and blue (B) colors are sequentially arranged corresponding to each of the sub-pixels SP.
FIG. 3 is an enlarged view of a sub-pixel region SP of FIG. 2 according to the related art. In FIG. 3, the sub-pixel region SP (in FIG. 2) includes a light emission region, a TFT region, and a storage capacitor region. In the TFT region, a semiconductor layer 62, a gate electrode 68, and source and drain electrodes 80 and 82 are sequentially formed on a transparent substrate 1, thereby forming a thin film transistor T (in FIG. 2). A power electrode 72 extending from a power supply line (not shown) and an organic electroluminescent diode E are connected to the source electrode 80 and the drain electrode 82, respectively. In the storage capactor region, a capacitor electrode 64 is formed under the power electrode 72 using the same material as that of the semiconductor layer 62, and an insulating layer is disposed between the power electrode 72 and the capacitor electrode 64, wherein the capacitor electrode 64, the insulating layer, and the power electrode 72 form a storage capacitor. In the light emission region, the organic electroluminescent diode E has first and second electrodes 12 and 16, and an organic light-emitting layer 14 interposed between the first and second electrodes 12 and 16.
FIG. 4 is a flow chart of a fabrication sequence of an OELD device according to the related art. In FIG. 4, a first step ST1 includes forming array elements, such as scan lines, signal lines, power lines, switching thin film transistors and driving thin film transistors, on a first substrate. The scan lines are formed on a transparent substrate extending along a first direction and the signal and power lines are formed on the transparent substrate extending along a second direction perpendicular to the first direction, wherein the signal and power lines cross the scan lines and are spaced apart from each other. In addition, each of the switching thin film transistors are formed near intersections of the scan and signal lines, and each of the driving thin film transistors are formed near intersections of the scan and power lines.
A second step (ST2) includes patterning a first electrode, which is a first component of an organic electroluminescent diode and is connected to the driving thin film transistor, within each sub-pixel region.
A third step (ST3) includes forming an organic light-emitting layer, which is a second component of the organic electroluminescent diode, on the first electrode. If the first electrode functions as an anode electrode, the organic light-emitting layer may be formed in a sequence of a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer from a top surface of the first electrode.
A fourth step (ST4) includes forming a second electrode, which is a third component of the organic electroluminescent diode, on the light-emitting layer, wherein the second electrode is formed on an entire surface of the first substrate to function as a common electrode.
A fifth step (ST5) includes encapsulating the first substrate with a second substrate to protect the first substrate from external impact and to protect the organic light-emitting layer from being damaged by an infiltration of exterior air. Thus, an absorbent desiccant is further formed in an inner surface of the second substrate.
The bottom emission-type OELD devices are completed by attaching the encapsulated substrate upon which the array element layer and the organic electroluminescent diode are formed to an additional encapsulating substrate. If the array element layer and the organic electroluminescent diode are formed on the same substrate, then panel yield is dependent upon the product of the individual yields of the array element layer and the organic electroluminescent diode. However, the panel yield is greatly affected by the yield of the organic electroluminescent diode. Accordingly, if a defective organic electroluminescent diode is fabricated, which usually is caused by formation of thin films having thicknesses of 1000 Å contaminated by impurities, the panel is classified as an inferior panel. Thus, production costs and materials are lost, thereby decreasing the panel yield.
The bottom emission-type organic OELD devices are advantageous because of their high image stability and variable fabrication processing. However, the bottom emission-type OELD devices are not adequate for implementation in devices that require high resolution due to limitations of increased aperture ratios. In addition, since top emission-type OELD devices emit light upward through the substrate, the light can be emitted without undue influence by the thin film transistor that is positioned under the light-emitting layer. Accordingly, design of the thin film transistor may be simplified. In addition, the aperture ratio can be increased, thereby increasing an operational life span of the OELD device. However, since a cathode is commonly formed over the organic light-emitting layer in the top emission-type OELD devices, material selection and light transmittance are limited such that light transmission efficiency is lowered. If a thin film-type passivation layer is formed to prevent a reduction of the light transmittance, the thin film passivation layer may fail to prevent infiltration of exterior air into the device.